1. Field of the Invention
The present invention is related to the field of sampling liquefied natural gas (LNG).
The invention relates in particular to a sampling device.
Such a device will find a very particular application in the loading and unloading of methane carriers, the liquefaction trains, the re-condensers and any sampling device or method representative of LNG. Said device will be used for measuring the upper calorific value (by means of an in-line chromatograph or calorimeter), its Wobbe index, its composition for calculating the density of the liquid and gaseous states and for detecting eventual pollutants. In brief, the invention permits to take a sample of gas in order to check the quality and to calculate the energy in-line during the transactional transfers.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
In a known way, the natural gas is condensed, for its transport, to the liquid state through a cryogenic method at a temperature in the range of −160 degrees Celsius.
Under such circumstances, heat-insulation problems arise. The known devices possess insulation means that are generally used in cryogenics. A known example of insulation consists in wrapping the sampling device in a glass-wool sheathing having a thickness of several millimeters, namely 160 mm for a K factor of 18.7 milliwatt per Kelvin-meter. Such an insulation is however not fully satisfactory.
In order to improve it, it has also been devised to exert a vacuum at the level of the sheathing of the sampling probe. Thus, putting this sheathing of the probe under vacuum permits an enhanced heat insulation, in order to reduce the risks of fractioning of the gas. Such a solution does however not completely avoid the losses due to conduction and convection. In addition, this technique does not avoid at all the losses due to radiation.
Furthermore, this degree of heat insulation is not sufficient within the framework of LNG sampling, so that it entails a risk of fractioning of the gas in the sampling line, namely because of an enthalpy absorption inducing a temperature of the sample higher than the temperature of under-cooling of the LNG. In brief, the existing devices have trouble in vaporizing the gas molecules in a complete bond, resulting into an erroneous representativity of the samplings being made and a random error beyond limits during the calculations of the upper calorific value and the densities of the liquid and gas phases based on the chromatograms being obtained.
Indeed, in the existing devices, the sampled gas is conveyed, at the outlet of the probe, through a heated coil, inside and alongside which it is suddenly vaporized. Therefore, it is not possible to accurately control the vaporization and to limit the risks of fractioning.
In addition, such a coil does not ensure a control of the change from liquid phase into gas phase of the sampling under complete vaporization conditions. A mixed phase can be present at the outlet of the coil, which also does not guarantee a complete vaporization of the sample and the performances required by the standards in force, namely EN 12838: i.e. a random error lower than 54 16.4561 Kj/Kg for the upper calorific value, 18×10−4 Kg/m3 for the density of the sample in gas phase and 0.9 Kg/m3 for the density of the sample in liquid phase.
An example of an existing device for sampling and vaporizing Liquefied Natural Gas is described in US 2009/151427 and comprises a sampling probe located at an end of a circuit conveying the sampled gas to measuring means through a vaporization chamber. The gas sample taken is then evaporated directly at its inlet pressure into the vaporization chamber, without further energy transformation. This vaporization unavoidably causes a fractioning. The presence of this fractioning is explicitly shown by the presence of an accumulator capable of storing the natural gas vapors. Such an accumulator is aimed at mixing the gas vapors at the vaporization outlet with the existing and already stored gas vapors. This is a homogenization intent in order to limit the fractioning that took place during the vaporization.
Let's remind here that when a fractioning occurred there is a presence of mixed gas and liquid phases. These phases will circulate at different speeds. In particular, nitrogen and methane will evaporate first, producing gas pockets in the stream of the liquid phase. Therefore, the evaporation of the still liquid residue generates a higher measure of the other, heavier, components with intermittent bubbles of nitrogen (N2) and methane (CH4).
Therefore, such a device results into a non-uniformity of the measurement of nitrogen, which is one of the main pollutants looked for and the content of which must be obtained accurately. On the other hand, methane is the main component, the rate of which must also be obtained with certainty.
The accumulator thus intervenes for homogenizing the gas phases after vaporization and fractioning. However, the extreme difference between the molecular masses of the various components limits the possible homogenization of nitrogen and cannot achieve a 100% efficiency for methane. Finally, the vaporized gas sampling is unavoidably dissociated, even after passing through this accumulator.
In addition, a flow reducer is provided for, followed by releasing means, but causes a gradual pressure reduction, but weakens the liquid phase and causes the fractioning. It is then not possible to achieve a vaporization free of any fractioning.
Furthermore, the circuit passes through two check valves and is surrounded by a metallic partition without heat-insulation surrounding the explosion-proof box, inside which the heating cartridges are mounted in order to vaporize the sample. Such an arrangement unavoidably generates heat bridges upstream of the vaporization, causing an uncontrolled enthalpy absorption. These thermal losses induce a temperature of the sample higher than the temperature of the under-cooling of the LNG, causing inexorably a fractioning of the sample.